linux-kernel - Re: [PATCH] Documentation: atomic_t.txt: Explain ordering provided by smp_mb__{before,after}

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Date:   Fri, 19 Apr 2019 10:54:50 -0700
From:   "Paul E. McKenney" <paulmck@...ux.ibm.com>
To:     Alan Stern <stern@...land.harvard.edu>
Cc:     LKMM Maintainers -- Akira Yokosawa <akiyks@...il.com>,
        Andrea Parri <andrea.parri@...rulasolutions.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Daniel Lustig <dlustig@...dia.com>,
        David Howells <dhowells@...hat.com>,
        Jade Alglave <j.alglave@....ac.uk>,
        Luc Maranget <luc.maranget@...ia.fr>,
        Nicholas Piggin <npiggin@...il.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Will Deacon <will.deacon@....com>,
        Kernel development list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] Documentation: atomic_t.txt: Explain ordering provided
 by smp_mb__{before,after}_atomic()

On Fri, Apr 19, 2019 at 01:21:45PM -0400, Alan Stern wrote:
> The description of smp_mb__before_atomic() and smp_mb__after_atomic()
> in Documentation/atomic_t.txt is slightly terse and misleading.  It
> does not clearly state that these barriers only affect the ordering of
> other instructions with respect to the atomic operation.
> 
> This improves the text to make the actual ordering implications clear,
> and also to explain how these barriers differ from a RELEASE or
> ACQUIRE ordering.
> 
> Signed-off-by: Alan Stern <stern@...land.harvard.edu>

Queued for further review, thank you, Alan!

							Thanx, Paul

> ---
> 
> 
>  Documentation/atomic_t.txt |   11 ++++++++---
>  1 file changed, 8 insertions(+), 3 deletions(-)
> 
> Index: usb-devel/Documentation/atomic_t.txt
> ===================================================================
> --- usb-devel.orig/Documentation/atomic_t.txt
> +++ usb-devel/Documentation/atomic_t.txt
> @@ -171,7 +171,10 @@ The barriers:
>    smp_mb__{before,after}_atomic()
>  
>  only apply to the RMW ops and can be used to augment/upgrade the ordering
> -inherent to the used atomic op. These barriers provide a full smp_mb().
> +inherent to the used atomic op. Unlike normal smp_mb() barriers, they order
> +only the RMW op itself against the instructions preceding the
> +smp_mb__before_atomic() or following the smp_mb__after_atomic(); they do
> +not order instructions on the other side of the RMW op at all.
>  
>  These helper barriers exist because architectures have varying implicit
>  ordering on their SMP atomic primitives. For example our TSO architectures
> @@ -195,7 +198,8 @@ Further, while something like:
>    atomic_dec(&X);
>  
>  is a 'typical' RELEASE pattern, the barrier is strictly stronger than
> -a RELEASE. Similarly for something like:
> +a RELEASE because it orders preceding instructions against both the read
> +and write parts of the atomic_dec(). Similarly, something like:
>  
>    atomic_inc(&X);
>    smp_mb__after_atomic();
> @@ -227,7 +231,8 @@ strictly stronger than ACQUIRE. As illus
>  
>  This should not happen; but a hypothetical atomic_inc_acquire() --
>  (void)atomic_fetch_inc_acquire() for instance -- would allow the outcome,
> -since then:
> +because it would not order the W part of the RMW against the following
> +WRITE_ONCE.  Thus:
>  
>    P1			P2
>  
>